An organic EL element can manufacture a display element and lighting which are driven by low power, are thin and light, and have excellent flexibility, and has been therefore actively studied as a next generation light emitting display element.
An organic EL element has a structure having a pair of electrodes composed of a positive electrode and a negative electrode, and a single layer or a plurality of layers which are disposed between the pair of electrodes and contain an organic compound. Examples of a layer containing an organic compound include a light emitting layer and a charge transport/injection layer for transporting or injecting a charge such as a hole or an electron. As a method for forming these organic layers, a vacuum deposition method or a wet film formation method is used.
The vacuum deposition method is advantageous in that a high-quality film can be formed uniformly on a substrate, a luminescent element which can be easily laminated and has excellent characteristics can be easily obtained, an extremely small amount of impurities derived from a manufacturing process are mixed, and the like. Many organic EL elements which are practically used now are obtained by the vacuum deposition method using a low molecular weight material. Meanwhile, a vacuum deposition apparatus used in the vacuum deposition method has such problems that the apparatus is generally expensive, continuous manufacturing is difficult, and manufacturing cost is high when all the steps are performed in vacuum.
On the other hand, the wet film formation method does not require a vacuum process, does not require an expensive vacuum deposition apparatus, and therefore makes it possible to form a layer at relatively low cost. In addition, the wet film formation method is advantageous in that an area can be large, continuous manufacturing is possible, a plurality of materials having various functions can be contained in one layer (coating liquid), and the like. Meanwhile, in the wet film formation method, lamination is difficult, and it is difficult to obtain a high-quality and uniform coating film which does not contain impurities derived from a manufacturing process (for example, a solvent).
In particular, development of an ink for forming a hole injection layer, a hole transport layer, and a light emitting layer has been positively carried out for material development for a wet film formation method. Among these developments, regarding inks for a hole injection layer and a hole transport layer, characteristics of each layer formed by the wet film formation method using these inks have reached a practical level. Regarding an ink for forming a light emitting layer, development of inks for a red light-emitting layer and a green light-emitting layer is progressing toward improvement of characteristics. However, regarding an ink for a blue light-emitting layer, in general, development of a composition using a polycyclic aromatic compound having an aromatic ring, such as anthracene, a styryl derivative, or the like has been carried out, but has not obtained practical characteristics. In particular, at present, an ink for a blue light-emitting layer having high color purity has not been developed.